Charge-changing energy loss, higher-orderZ1dependence, and pre-equilibrium behavior in the stopping power for energetic ions in solids

Relative stopping cross sections are measured for 3-MeV/u He, Li, and C ions in 2-26-μg/${\mathrm{cm}}^2$ C foils. The observed stopping cross sections for the fully stripped He and Li ions scale essentially as $Z_1^2$, in agreement with standard theory on higher-order $Z_1$-dependent effects applied to C targets. We observe nonequilibrium charge-state effects in the energy loss of C ions. Postfoil charge-state selection is used to measure the fixed-charge stopping cross sections for the fully stripped and one-electron C ions, and to extract the energy lost in the charge-change cycle ${\mathrm{C}}^{6+}$ ⇄ ${\mathrm{C}}^{5+}$ with the aid of a two-component charge-state model. The results show that charge-changing energy loss forms a significant part of the total stopping for 3-MeV/u C ions, and a significant higher-order $Z_1$-dependent effect is seen in the stopping cross section for ${\mathrm{C}}^{6+}$ ions. The standard interpretation in terms of the Bloch term and polarization theory is discussed in the light of these observations.